Cycloaliphatic polyester polyols are intermediates used for the manufacture of polyurethane products, including foams, coatings, sealants, adhesives, and elastomers. The cycloaliphatic nature of these polyols imparts strength and flexibility while also providing desirable color stability, especially for coatings.
Cycloaliphatic polyester polyols can be made by hydrogenating aromatic rings of corresponding aromatic polyester polyols, which are in turn made by condensing aromatic diacid, diesters, or anhydrides (e.g., terephthalic acid, dimethyl terephthalate) with glycols such as ethylene glycol, propylene glycol, diethylene glycol, or the like. These starting materials usually derive exclusively from petrochemical sources.
As companies increasingly seek to offer products with improved sustainability, the availability of intermediates produced from bio-renewable and/or recycled materials becomes more leveraging. However, there remains a need for these products to deliver equal or better performance than their traditional petroleum-based alternatives at a comparable price point.
Bio-renewable content alone can be misleading as an indicator of “green” chemistry. For example, when a food source such as corn is needed to provide the bio-renewable content, there are clear trade-offs between feeding people and providing them with performance-based chemical products. Additionally, in some cases, the chemical or biochemical transformations needed to convert sugars or other bio-friendly feeds to useful chemical intermediates such as polyols can consume more natural resources and energy and can release more greenhouse gases and pollutants into the environment than their petro-based alternatives in the effort to achieve “green” status.
Waste thermoplastic polyesters, including waste polyethylene terephthalate (PET) streams (e.g., from plastic beverage containers), provide an abundant source of raw material for making new polymers, including cycloaliphatic polyester polyols. Usually, when PET is recycled, it is used to make new PET beverage bottles, PET fiber, or it is chemically transformed to produce polybutylene terephthalate (PBT). Other recycled raw materials are also available. For example, recycled propylene glycol is available from aircraft or RV deicing and other operations, and recycled ethylene glycol is available from spent vehicle coolants.
Urethane formulators demand polyols that meet required specifications for color, clarity, hydroxyl number, functionality, acid number, viscosity, and other properties. These specifications will vary and depend on the type of urethane application.
Polyols suitable for use in making high-quality polyurethanes have proven difficult to manufacture from recycled materials, including recycled polyethylene terephthalate (rPET). Many references describe digestion of rPET with glycols (also called “glycolysis”), usually in the presence of a catalyst such as zinc or titanium. Digestion converts the polymer to a mixture of glycols and low-molecular-weight PET oligomers. Although such mixtures have desirably low viscosities, they often have high hydroxyl numbers or high levels of free glycols. Frequently, the target product is a purified bis(hydroxyalkyl) terephthalate (see, e.g., U.S. Pat. Nos. 6,630,601, 6,642,350, and 7,192,988) or terephthalic acid (see, e.g., U.S. Pat. No. 5,502,247). Some of the efforts to use glycolysis product mixtures for urethane manufacture are described in a review article by D. Paszun and T. Spychaj (Ind. Eng. Chem. Res. 36 (1997) 1373.
Most frequently, ethylene glycol is used as the glycol reactant for glycolysis. This is sensible because it minimizes the possible reaction products. Usually, the glycolysis is performed under conditions effective to generate bis(hydroxyethyl) terephthalate (“BHET”), although sometimes the goal is to recover pure terephthalic acid. When ethylene glycol is used as a reactant, the glycolysis product is typically a crystalline or waxy solid at room temperature. Such materials are less than ideal for use as polyol intermediates because they must be processed at elevated temperatures. Polyols are desirably free-flowing liquids at or close to room temperature.
U.S. Pat. Nos. 6,359,022 and 6,664,363 teach to use hydrophobic materials, including fatty acids, fatty acid methyl esters, and triglycerides (fats and oils) as reactive components for making aromatic polyester polyols. The hydrophobic materials provide polyols with reduced viscosity at a given hydroxyl number and improved hydrocarbon solubility compared with traditional polyester polyols. The modified aromatic polyester polyols can be used more effectively with pentane and other blowing agents used to make rigid polyurethane foams. A wide variety of hydrophobic materials are taught as suitable for use. Scrap PET is taught as a useful alternative starting material to the usual phthalic anhydride reactant, but the working examples are limited to phthalic anhydride-based polyester polyols. Recently, we showed that aromatic polyester polyols can be made by reacting thermoplastic polyesters (e.g., recycled PET), glycols, and various hydrophobes (e.g., vegetable oils, dimer fatty acids, cardanol, and the like). The resulting polyols have high recycle content and desirable attributes for formulating polyurethane products.
Improved polyols are needed. In particular, the urethane industry needs sustainable polyols based in substantial part on recycled polymers such as the practically unlimited supply of recycled polyethylene terephthalate. Cycloaliphatic polyester polyols with high recycle content that satisfy the demanding color, clarity, viscosity, functionality, and hydroxyl content requirements of polyurethane formulators would be valuable.